The invention relates to materials for use as coatings and adhesives that may be disbanded from a surface to which they are applied without harm to that surface. The invention further relates to methods of disbonding adhesives and coatings from substrate surfaces.
Adhesive bonds and polymeric coatings are commonly used in the assembly and finishing of manufactured goods. Adhesive bonds are used in place of mechanical fasteners, such as screws, bolts and rivets, to provide bonds with reduced machining costs and greater adaptability in the manufacturing process. Adhesive bonds distribute stresses evenly, reduce the possibility of fatigue, and seal the joints from corrosive species. Similarly, polymer-based coatings are commonly applied to the exterior surface of manufactured products. These coatings provide protective layers that seal the surface from corrosive reactants, as well as provide a painted surface that can be aesthetically pleasing.
Among the best adhesives and coatings in terms of strength and durability are those based on thermosetting polymers. Typically applied as a liquid mixture of low molecular weight monomers, these adhesives wet and penetrate pores on the substrate surface. On cure, insoluble and infusible crosslinked polymers are formed that are mechanically interlocked and often covalently bound to the substrate to which they are applied. Common amine-cured epoxies are a typical example of adhesives and coatings that employ thermosetting mixtures.
Although adhesive bonds offer many advantages over mechanical fasteners, adhesive bonds are essentially permanent. There are no methods available for ready disassembly of adhesively bonded objects. The separation strategies that do exist typically involve time-consuming chemical procedures requiring high temperatures and aggressive chemicals. Examples of such techniques are described in U.S. Pat. No. 4,171,240 by Wong and U.S. Pat. No. 4,729,797 by Linde et al. These techniques, although generally effective, are quite harsh and can damage the objects being separated, making them unsuitable for many applications.
Similarly, conventional coating materials, such as polyurethanes, epoxies, phenolics, melamines, and the like, are essentially permanent. Such coatings are often removed with an aggressive chemical agent that is applied to the coating surface to degrade the coating material. Mechanical abrasion, such as sand blasting or wire brushing, is also employed. Although these techniques are effective in removing the polymer coating, they are time and labor intensive, as well as being quite harsh and likely to cause damage to the underlying surface.
To provide materials that are more easily removed from a substrate, the prior art describes adhesives formed from reactive monomers containing linkages susceptible to chemical degradation. Such materials are described in U.S. Pat. Nos. 5,512,613 and 5,560,934 to Afzali-Ardakani et al. and in U.S. Pat. No. 4,882,399 to Tesoro et al. Additionally, S. Yang et al in xe2x80x9cReworkable Epoxies: Thermosets with Thermally Cleavable Groups for Controlled Network Breakdownxe2x80x9d, Chem. Mater. 10:1475 (1998) and Ogino et al. in xe2x80x9cSynthesis and Characterization of Thermally Degradable Polymer Networksxe2x80x9d, Chem. Mater. 10(12):3833 (1998) describe curable resins containing thermally labile linkages. Other polymers containing thermally labile or thermally reversible crosslinks are described in U.S. Pat. No. 3,909,497 to Hendry et al. and U.S. Pat. No. 5,760,337 to Iyer et al. An alternative approach to bond cleavage is described in U.S. Pat. No. 5,100,494 to Schmidt which discloses embedding a nichrome heating element within a thermoplastic so that the adhesive softens or melts upon resistive heating. Although these specially prepared materials are more readily cleaved from the substrate, they still require conditions that are harsh to delicate substrates or adjacent adhesive bonds.
Thus, there remains a need in the art for a material capable of being disbanded selectively and precisely under mild conditions. Such a material would provide adhesive bonds and coatings that could be employed in a variety of applications where facile removal of the material from the surface is desired.
The present invention provides a composition capable of strong, yet temporary, substrate bonding or coating that is removable without damage to the underlying substrate. It may be used in both temporary and permanent bonding and coating applications.
An electrochemically disbondable composition of the invention includes a matrix functionality and an electrolyte functionality. The matrix functionality provides an adhesive bond to a substrate, and the electrolyte functionality provides sufficient ionic conductivity to the composition to support a faradaic reaction at an interface with an electrically conductive surface in contact with the composition. The adhesive bond is weakened at the interface on application of an electrical potential across the interface. In preferred embodiments, the disbondable composition is a phase separated material having first regions of substantially matrix functionality and second regions of substantially electrolyte functionality.
The xe2x80x9cmatrix functionalityxe2x80x9d of a material is the ability of a material or a mixture of materials to join by mechanical or chemical bonding to a substrate and to adhere to the substrate by virtue of this bond. Matrix functionality also provides mechanical strength to the material, such that the material is capable of transferring load between substrates or, as a coating, is self-supporting.
The xe2x80x9celectrolyte functionalityxe2x80x9d of a material is the ability of the material to conduct ions, either anions, cations or both. The ions are provided by a salt added to the material or are chemically incorporated into the material as an ionomer, that is, a polymer containing ionized groups. The electrolyte functionality is understood to derive from the ability of the composition to solvate ions of at least one polarity.
The term xe2x80x9cfaradaic reactionxe2x80x9d means an electrochemical reaction in which a material is oxidized or reduced.
The term xe2x80x9cadhesivexe2x80x9d refers to polymer-based materials which are capable of holding materials together by surface attachment. An adhesive typically forms a bond to a substrate by mechanical interlocking and often covalent bonding to the substrate. The adhesive is chemically distinct from the bonded substrate and the bonded materials may be dissimilar from one another.
In one embodiment of the invention, the matrix functionality is provided by a polymer selected from the group consisting of epoxies, phenolics, acrylics, melamines, maleimides, and polyurethanes.
In another embodiment of the invention, the polymer has a variable crosslink density to form regions of low crosslink density having a relatively high ionic conductivity and regions of high crosslink density having a relatively high mechanical strength.
In another embodiment of the invention, the polymer includes coordination sites that are capable of solvating ions and that support the electrolyte functionality of the composition.
In other embodiments, the electrolyte functionality is provided by an electrolyte additive selected from the group consisting of ionically conductive monomers, oligomers and polymers, and ionomers and may be localized in regions within the polymer to form a secondary phase of high ionic conductivity and mobility.
In one preferred embodiment, the disbondable composition is an adhesive, and may have a lap shear strength in the range of 2000-4000 psi. In another preferred embodiment, the composition is a coating, and may be resistant to delamination from a substrate to which it is applied.
In another aspect of the invention, an electrochemically disbondable composition is provided having an uncured polymeric material having an electrolyte located therein. The uncured polymeric material, when cured, provides in combination with the electrolyte, sufficient solubility and mobility to the electrolyte to support a faradaic reaction at a surface in electrical contact with an electrode.
Another aspect of the invention includes a corrosion resistant coating in which a substrate subject to corrosion has as a coating a composition having a matrix functionality and an electrolyte functionality, the matrix functionality providing an adhesive bond to the substrate, and the electrolyte functionality providing sufficient ionic conductivity to the composition to support a faradaic reaction at an interface with the substrate. In yet another aspect of the invention, a bonded structure includes two electrically conductive surfaces, and a bond between the two surfaces composed of the electrochemically disbondable composition of the invention. The conductive surface may be an article or articles to be secured by the bond or they may be a conductive element selected from the group consisting of sheets, foils, grids and meshes. The conductive element further may be bonded to an article using a conventional adhesive or the disbondable composition of the invention.
In one embodiment of the invention, the electrically conductive surface is an electrically conductive coating applied to a substrate, which may be non-electrically conducting.
In another embodiment of the invention, the bonded structure includes first and second electrically conductive surfaces; and an electrically conductive element disposed therebetween. The electrochemically disbondable composition of the invention is used to bond the electrically conductive element to the first and second conductive surfaces.
A laminate structure is also provided which is particularly advantageous in the joining of irregular or non-conductive surfaces. The laminate includes first and second electrically conductive elements selected from the group consisting of foils, sheets, meshes and grids, and the disbondable composition of the invention disposed therebetween and bonded to the first and second elements.
In another aspect of the invention, a method is provided for disbanding a composition from an electrically conductive surface to which it is bonded. The method includes providing a first electrically conductive surface treated with the electrochemically disbondable composition of the invention, contacting a second electrically conductive surface to the composition, and passing an electric current through the disbondable composition to cause a faradaic reaction at the surface, whereby the bond to the surface is weakened.
The bond between the disbondable composition and a substrate may be weakened in a short time by the flow of electrical current across the bondline between the substrate and the composition. Typically, the bond is weakened sufficiently that the substrate is separated easily by hand from the disbondable composition. At least one of the substrates separates cleanly and is substantially free from any residual bonding composition. Because the disbanding procedure uses electricity instead of heat or chemical reagents, inadvertent disbanding during normal use is unlikely.
In yet another aspect of the invention, an electrochemically disbondable composition having a matrix functionality and an electrolyte functionality, the matrix functionality providing an adhesive bond to a substrate, and the electrolyte functionality providing sufficient ionic conductivity to the composition to support a faradaic reaction at an interface with an electrically conductive surface in contact with the composition is provided, wherein the electrolyte functionality includes a block copolymer or a graft copolymer.
In one embodiment, the graft polymer includes a siloxane backbone and pendant blocks of high ionic conductivity, and the pendent blocks may include polyether or polyglycol moieties. In another embodiment the graft copolymer includes dimethicone polyols. The graft copolymer may be selected from the group consisting of dimethyl-methyl(polyethylene oxide) siloxanes, alkoxylated 3-hydroxypropyl-terminated dimethyl siloxanes, and dimethyl-methyl(3-hydroxypropyl) siloxanes.
In another embodiment of the invention, the block copolymer includes a linear block copolymer having matrix miscible and matrix immiscible blocks. The matrix miscible blocks may be selected from the group consisting of aliphatic and aromatic polyethers, nitrile-functionalized polymers, acrylic and vinyl polymers and polyamides. The matrix immiscible blocks may be selected from the group consisting of aliphatic and aromatic hydrocarbon polymers, acrylic and vinyl polymers, silicone polymers, phosphazine polymers, fluoropolymers, polysulfides, polyesters, polyamides and rigid-rod polymers.
In one embodiment, the linear block copolymer includes polyethylene-block-poly(ethylene glycol), or poly(ethylene glycol)-block-poly(propylenene glycol)-block-poly(ethylene glycol).
In other embodiments, the matrix functionality is made up of a polymer selected from the group consisting of epoxies, phenolics, acrylics, melamines, maleimides, and polyurethanes, and combinations thereof.
In still other embodiments, the electrolyte functionality includes a salt capable of being solvated into the composition, such alkali metal, alkaline earth and ammonium salts. The salt may include an anion selected from the group consisting of hexafluorophosphate, tetrafluoroborate, hexafluoroantimonate, and perchlorate, or it may be selected singly or as a mixture from the group consisting of lithium trifluoromethane sulfonimide and lithium trifluromethane sulfonate.
In still other embodiments, the composition has an ionic conductivity in the range of 10xe2x88x9211 to 10xe2x88x925 S/cm2; may further comprise an additive selected from the group consisting of pigments, corrosion inhibitors, leveling agents, gloss promoters, rubber tougheners and fillers; or may be an adhesive or coating.
In another aspect of the invention, an electrochemically disbondable composition is provided having a curable material having an electrolyte located therein, wherein electrolyte functionality is provided by a block copolymer or a graft copolymer. The uncured polymeric material, when cured, provides in combination with the electrolyte, sufficient ionic conductivity to support a faradaic reaction at a surface in electrical contact with an electrode. The curable material may be selected from the group consisting of epoxy resins, phenolic resins, acrylic resins, melamine resins, malemide resins, and urethanes.
In still another aspect of the invention, a bonded structure includes two electrically conductive surfaces and a bond between the two surfaces. The bond includes an electrochemically disbondable composition having a matrix functionality and an electrolyte functionality, the matrix functionality providing an adhesive bond to a substrate, and the electrolyte functionality providing sufficient ionic conductivity to the composition to support a faradaic reaction at an interface with an electrically conductive surface in contact with the composition, wherein the electrolyte functionality includes a block copolymer or a graft copolymer.
In still another aspect of the invention, a method for disbonding a composition from an electrically conductive surface to which it is bonded is provided. The method includes providing a first electrically conductive surface treated with an electrochemically disbondable composition having a matrix functionality and an electrolyte functionality, wherein the electrolyte functionality is provided by a block copolymer or a graft copolymer. The matrix functionality provides an adhesive bond to the first conductive surface, and the electrolyte functionality provides sufficient ionic conductivity to the disbondable composition to support a faradaic reaction at an interface of the first conductive surface and the disbondable composition. A second electrically conductive surface is contacted to the disbondable composition, and a voltage is applied across the disbondable composition to cause a faradaic reaction at the first conductive surface, whereby the bond to the first conductive surface is weakened.